Box Fill Calculations

Box Fill Calculations

By Mike Holt for EC&M Magazine

Calculate conductor equivalents to prevent box fill violations.

Box fill isn’t just the number of wires in the box. It’s the total volume of the conductors, devices, and fittings in a box.

Every outlet box has a specific amount of space for conductors, devices, and fittings—we call that the box volume. You calculate box volume per 314.16(A) and box fill per 314.16(B). You need to make sure your 314.16(A) box volume is greater than or equal to your 314.16(B) box fill.

Box volume

Table 314.16(A) seems to make it very simple to determine the volume of a box. But, don’t get too excited. You can use Table 314.16(A) only if the box contains no switches, receptacles, luminaire studs, luminaire hickeys, internal cable clamps, or equipment grounding conductors. This is rarely the case.

If Table 314.16(A) is inapplicable for your installation, which it typically will be, how do you calculate box volume? It’s a matter of adding up individual volumes of assembled parts [314.16(A)].

Start by with the box itself. If the box is not a standard size, the volume will be marked on the box by the manufacturer [314.16(A)(2)]. Then, add to it the sum of the individual volumes of the assembled parts, such as plaster rings, extension rings, etc. Include only those parts that are marked with their volumes in cubic inches [314.16(A)] or included in NEC Table 314.16(A).

Conductor volume

Calculating conductor volume is a matter of adding up individual conductor equivalent volumes-- and there are five such volumes. After you calculate all five volumes using Table 314.16(B), add up them up. The number you get is the total conductor volume. In no case can this exceed the box volume [314.16].

In this process, you don’t need to count raceway and cable fittings (including locknuts and bushings), wire connectors, or cable connectors with their clamping mechanism outside of the box. Nor do you need to count conductors that originate and terminate within the outlet box (such as equipment bonding jumpers and pigtails) [314.16(A)].

Using Table 314.16(B), calculate each of the five following conductor equivalent volumes:

(1) Conductor Volume. Each unbroken conductor that runs through a box, as well as each conductor that terminates in a box, is counted as a single conductor volume. Each loop or coil of unbroken conductor having a length of at least twice the minimum length required for free conductors in 300.14 must be counted as two conductor volumes. Conductors that originate and terminate within the box (e.g., pigtails) aren’t counted at all.

You can omit equipment grounding conductors, and up to four 16 AWG and smaller fixture wires, from box fill calculations if they enter the box from a domed luminaire or similar canopy (e.g., a ceiling paddle fan canopy) [413.16(B)(1) Ex].

(2) Cable Clamp Volume. One or more internal cable clamps count as a single conductor volume, based on the largest conductor that enters the box. Cable connectors that have their clamping mechanism outside the box aren’t counted.

(3) Support Fitting Volume. Each luminaire stud or luminaire hickey counts as a single conductor volume, based on the largest conductor that enters the box.

(4) Device Yoke Volume. Each single gang device yoke (regardless of the ampere rating of the device) counts as two conductor volumes, based on the largest conductor that terminates on the device. Each multigang device yoke counts as two conductor volumes for each gang, based on the largest conductor that terminates on the device.

A device that’s too wide for mounting in a single gang box, as described in Table 314.16(A), is counted based on the number of gangs required for the device.

5) Equipment Grounding Conductor Volume. All equipment grounding conductors in a box count as a single conductor volume, based on the largest equipment grounding conductor that enters the box. Insulated equipment grounding conductors for receptacles having insulated grounding terminals (isolated ground receptacles) [250.146(D)] count as an additional single conductor volume.

Total conductors

What is the total number of conductors used for the box fill calculations in Figure 5–27?

(a) 5 conductors (b) 7 conductors

Answer: (d) 11 conductors

Switch and conductors 5 – 14 AWG †

Receptacles and conductors 4 – 14 AWG ††

Equipment grounding conductor 1 – 14 AWG

Cable clamps + 1 – 14 AWG

Total 11 – 14 AWG

†two conductors for the device and three conductors terminating

††two conductors for the device and two conductors terminating

Each 14 AWG counts as 2 cu in. [Table 314.16(B)].

11 conductors x 2 cu in. = 22 cu in.

If the cubic inch volume of the mud ring is not stamped on it, or given in the problem, we cannot include it in the box volume. Without knowing the mud ring volume, a 4 in. square by 21⁄8 in. deep box would be the minimum required for this example.

Box Sizing

To determine the size of the outlet box when the conductors are of different sizes, follow these steps:

Step 1: Determine the number and size of conductor equivalents in the box.

Step 2: Determine the volume of the conductor equivalents from Table 314.16(B).

Step 3: Size the box by using Table 314.16(A).

Let’s work an example. What is the minimum size square outlet box required for one 14/3 Type NM cable that terminates on a 3-way switch, and one 12/2 Type NM cable that terminates on a receptacle? The box has internally installed cable clamps.

(a) 4 x 1¼ square (b) 4 x 1½ square

Answer: (c) 4 x 21/8 square

Step 1: Determine the number of each size conductor.

14 AWG

14/3 NM = 3 – 14 AWG

Switch + 2 – 14 AWG

Total 5 – 14 AWG

12 AWG

12/2 NM 2 – 12 AWG

Cable clamp 1 – 12 AWG

Receptacle 2 – 12 AWG

Equipment grounding conductor + 1 – 12 AWG

Total 6 – 12 AWG

All equipment grounding conductors count as one conductor, based on the largest equipment grounding conductor entering the box [314.16(B)(5)].

Step 2: Determine the volume of the conductors [Table 314.16(B)].

14 AWG 2 cu in. each

2 cu in. x 5 conductors 10 cu in.

12 AWG 2.25 cu in. each

2.25 cu in. x 6 conductors 13.50 cu in.

Total Volume 10 cu in. + 13.50 cu in.

Total Volume 23.50 cu in.

Step 3: Select the outlet box from Table 314.16(A).

4 x 21/8 square, 30.30 cu in. meets the minimum cu in. requirements.

Getting it right

Sometimes, you already have a box in place and need to make sure you don’t overfill it. This could easily be the case during remodeling/retrofit work, or following a change order on new construction. Other times, you know how many conductors will go in a given run and you need to make sure you install a big enough box.

Either way, you calculate box volume and conductor volume. But you can summarize the whole process this way:

If conductors are the same size, add them together and size the box using the AWG size columns of Table 314.16(A). Your conductor equivalents will all be the same size as the conductors.
If the box contains different sizes of conductors, use Table 314.16(B) to find the area of each conductor, add them up, and size the box from Table 314.16(A) using the cu in. column.

Remember:

Calculating box volume is a matter of adding up individual volumes of assembled parts [314.16(A)].
Calculating conductor volume is a matter of adding up individual conductor equivalent volumes [314.16(B)].

If box fill exceeds box volume, you need to reduce the number of conductors or use a larger box.

Keep in mind that these are minimum requirements [90.1]. There’s no penalty for using a box that’s too big, other than the additional cost of that box versus a smaller one. It’s typically more cost-effective on a project to use a smaller quantity of box sizes and have a few that are over-sized, than to calculate the exact minimum needed at every point and try to match them all up in the field.